Saturday, October 1, 2011

Equals and hashcode methods are two primary but yet one of most important methods for java developers to be aware of. Java intends to provide equals and hashcode for every class to test the equality and to provide a hash or digest based on content of class. Importance of hashcode increases when we use the object in different collection classes which works on hashing principle e.g. hashtable and hashmap. A well written hashcode method can improve performance drastically by distributing objects uniformly and avoiding collision. In this article we will see how to correctly override hashcode() method in java with a simple example. We will also examine important aspect of hashcode contracts in java. This is in continuation of my earlier post on overriding equals method in Java, if you haven’t read already I would suggest go through it.

General Contracts for hashCode() in Java

1) If two objects are equal by equals() method then there hashcode returned by hashCode() method must be same.

2) Whenever hashCode() mehtod is invoked on the same object more than once within single execution of application, hashCode() must return same integer provided no information or fields used in equals and hashcode is modified. This integer is not required to be same during multiple execution of application though.

3) If two objects are not equals by equals() method it is not require that there hashcode must be different. Though it’s always good practice to return different hashCode for unequal object. Different hashCode for distinct object can improve performance of hashmap or hashtable by reducing collision.

To better understand concept of equals and hashcode and what happens if you don’t override them properly I would recommend understanding of How HashMap works in Java

Overriding hashCode method in Java

We will follow step by step approach for overriding hashCode method. This will enable us to understand the concept and process better.

1) Take a prime hash e.g. 5, 7, 17 or 31 (prime number as hash, results in distinct hashcode for distinct object)

2) Take another prime as multiplier different than hash is good.

3) Compute hashcode for each member and add them into final hash. Repeat this for all members which participated in equals.

4) Return hash

Here is an example of hashCode() method

@Override

public int hashCode() {

int hash = 5;

hash = 89 hash + (this.name != null ? this.name.hashCode() : 0);

hash = 89 hash + (int) (this.id ^ (this.id >>> 32));

hash = 89 hash + this.age;

return hash;

}

It’s always good to check null before calling hashCode() method on members or fields to avoid NullPointerException, if member is null than return zero. Different data types has different way to compute hashCode.Integer members are simplest we just add there value into hash, for other numeric data-type are converted into int and then added into hash. Joshua bloach has full tables on this. I mostly relied on IDE for this.

Better way to override equals and hashCode

In my opinion better way to override both equals and hashcode method should be left to IDE. I have seen Netbeans and Eclipse and found that both has excellent support of generating code for equals and hashcode and there implementations seems to follow all best practice and requirement e.g. null check , instanceof check etc and also frees you to remember how to compute hashcode for different data-types.

Let’s see how we can override hashcode method in Netbeans and Eclipse.

In Netbeans

1) Write your Class.

2) Right click + insert code + Generate equals() and hashCode().

In Eclipse

1) Write your Class.

2) Go to Source Menu + Generate hashCode() and equals()

Things to remember while overriding hashcode in Java

1. Whenever you override equals method, hashcode should be overridden to be in compliant of equals hashcode contract.

2. hashCode() is declared in Object class and return type of hashcode method is int and not long.

3. For immutable object you can cache the hashcode once generated for improved performance.

4. Test your hashcode method for equals hashcode compliance.

5. If you don'toverride hashCode() method properly your Object may not function correctly on hash based collection e.g. HashMap, Hashtable or HashSet.

Writing equals and hashcode using Apache Commons EqualsBuilder and HashCodeBuilder

EqualsBuilder and HashCodeBuilder from Apache commons are much better way to override equals and hashcode method, at least much better than ugly equals, hashcode generated by Eclipse. I have written same example by using HashCodebuilder and EqualsBuilder and now you can see how clear and concise they are.

@OnlySoftware , thanks for your suggestion I tried both EqualsBuilder and HashCodeBuilder and found them easy to use and probably best option, though it introduce additional dependency on Apache commons but in the end its worth to include Apache commons which has lots of utility.

Can we make hashCode method synchronized in Java, What are the consequences of using synchronized keyword with equals and hashCode method ? Also is there any way we can still do in memory comparison using native equals method in Java

hashcode in java example is very good but you didn't tell us how to write hashcode for immutable class, will be same of there would be any difference. also what are performance consideration in java while overriding hashcode in java ? What happend if I return same hashcode for every object ?

Use IDE to generate code for equals and hashCode, there is no point doing it manually. you might want to highlight the importance of hashCode when an object is used as key in HashMap. Though its not a requirement that unequal object should have unequal or unique hashCode, you should try for that.

Hi Nice article, In one of my interviews asked me a question that what happens when hashcode of two key objects are equals? tell me what happens while put and get methods. I couldn't answer this question. I came back and written a simple program like for all custom objects it returns the same hashcode, tried adding them to hashmap. But it is working fine, I mean to say I could see the size of the hashmap correctly though I gave same hashcode. Also I could get the values properly, I dont see any issues. I printed the hashcode for all the objects they are same only. So finally how will I know hash collision happened?

@Naveen Vanjani and saikinranputtaYou need to override equals and hashcode if you want colletions based on hashes to work well, like hashset and hashmap.

If you put a Person object with the same 'name' and 'surname' fields inside a HashSet many times you'd expect the set to only have it once, but as their hashcodes wouldn't be the same without overriding hashcode() you'd have it repeated.

And equals is needed because sometimes in HashMaps for example there are hashcode collisions, and when that happens the objects are put in the same bucket (linkedlist). Then to look for the right object the equals method is used.

@Venkat SarmaProbably you did override hashcode() without overriding equals. So in that case all your objects were stored in the same bucket, which leads to O(n) lookup instead of O(1) which is normally expected in a HashMap.